Field of the Invention
Embodiments of the present invention relate generally to a method for measuring an amount of pattern width deviation, and to a pattern inspection apparatus. More specifically, embodiments of the present invention relate, for example, to a pattern width deviation measuring method of an inspection apparatus which inspects a pattern by radiating a light beam to acquire an optical image of the pattern image.
Description of Related Art
In recent years, with the advance of high integration and large capacity of large-scale integration (LSI) circuits, the line width (critical dimension) required for circuits of semiconductor elements is becoming progressively narrower. Such semiconductor elements are manufactured by circuit formation of exposing and transferring a pattern onto a wafer by means of a reduced proj ection exposure apparatus known as a stepper while using an original or “master” pattern (also called a mask or a reticle, hereinafter generically referred to as a mask) with a circuit pattern formed thereon. Then, in fabricating a mask for transfer printing such a fine circuit pattern onto a wafer, a pattern writing apparatus using electron beams, capable of writing or “drawing” fine circuit patterns, needs to be employed. Pattern circuits may be written directly on the wafer by the pattern writing apparatus. Also, a laser beam writing apparatus that uses laser beams in place of electron beams for writing a pattern is under development.
Since LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. However, as typified by a 1-gigabit DRAM (Dynamic Random Access Memory), the scale of patterns configuring an LSI is in transition from on the order of submicrons to nanometers. One of major factors that decrease the yield of the LSI manufacturing is due to pattern defects on the mask used for exposing and transfer printing an ultrafine pattern onto a semiconductor wafer by the photolithography technology. In recent years, with miniaturization of dimensions of LSI patterns formed on a semiconductor wafer, dimension to be detected as a pattern defect has become extremely small. Therefore, a pattern inspection apparatus for inspecting defects on a transfer mask used in manufacturing LSI needs to be more highly accurate.
As an inspection method, there is known a method of comparing an optical image obtained by imaging a pattern formed on a target object or “sample” such as a lithography mask at a predetermined magnification by using a magnification optical system with design data or an optical image obtained by imaging the same pattern on the target object. For example, the methods described below are known as pattern inspection methods: the “die-to-die inspection” method that compares data of optical images of identical patterns at different positions on the same mask; and the “die-to-database inspection” method that inputs, into an inspection apparatus, writing data (design pattern data) generated by converting pattern-designed CAD data to a writing apparatus specific format to be input to the writing apparatus when a pattern is written on the mask, generates design image data (reference image) based on the input writing data, and compares the generated design image data with an optical image (serving as measurement data) obtained by imaging the pattern. In such inspection methods for use in the inspection apparatus, a target object is placed on the stage so that a light flux may scan the target object as the stage moves in order to perform an inspection. Specifically, the target object is irradiated with a light flux from the light source through the illumination optical system. Light transmitted through the target object or reflected therefrom forms an image on a sensor through the optical system. The image captured by the sensor is transmitted as measurement data to the comparison circuit. After performing position adjustment of images, the comparison circuit compares measurement data with reference data in accordance with an appropriate algorithm, and determines that there exists a pattern defect if the compared data are not identical.
In the pattern inspection, in addition to inspecting a pattern defect (shape defect), it is also required to measure a line width (CD, critical dimension) deviation of a pattern. Conventionally, measuring a pattern line width (CD) deviation has been performed using a dedicated measuring device. If this measurement can be simultaneously performed at the time of pattern defect inspection, a significant advantage cost-wise and inspection-time-wise can be achieved. Therefore, it is becoming increasingly requested for the inspection apparatus to have such a measurement function. Regarding measuring CD deviation, there is proposed an inspection method for measuring line width deviation of a pattern in an image acquired for each set region (for example, refer to Japanese Patent Application Laid-open (JP-A) No. 2014-181966).
In order to measure CD deviation by using the inspection apparatus, it is necessary to measure deviation of a pattern width formed on the real mask with respect to mask design data. Therefore, an optical image of a real mask needs to be captured. While moving the stage on which the real mask is placed, the image-capturing is performed. The focus position needs to be changed dynamically because it changes according to unevenness of a pattern. If auto-focus adjustment cannot follow the stage moving speed, following error of the focus position occurs. Since the profile of a captured image changes when a following error of the focus position arises, it becomes difficult to measure an accurate line width. As a result, error occurs in measuring CD deviation, and thus, there occurs a problem that the measurement accuracy degrades.
If the stage moving speed is decreased in order to reduce following errors of the focus position, the inspection time is prolonged, thereby degrading the throughput of the inspection apparatus. Accordingly, it is required to inhibit the degradation of the measurement accuracy without decreasing the stage moving speed.